The application of conductive material to inner surfaces of through holes, referred to as through hole printing, has been performed using manual printing systems. An operator hand loads a substrate onto a support referred to in the art as a printing nest. The operator locates the substrate on the printing nest so that the through holes on the substrate are aligned with corresponding holes in the printing nest. Holes in the printing nest being approximately the same diameter as the holes in the substrate. The operator then activates a hold-down vacuum, which is applied through holes in the printing nest located adjacent to the perimeter of the substrate, to clamp the substrate to the nest.
The print nest is thereafter moved into a printing position under a printing screen. The printing operation is then performed whereby the desired pattern of conductive paths, including the conductive material for the through holes, is printed on the substrate and the nest is moved back to the load/unload position. The operator then activates a second vacuum that is applied to the corresponding holes in the nest and serves to pull the conductive material through the holes in the substrate and coat the inner walls of the through holes. U.S. Pat. No. 4,710,395 issued to Young et al on Dec. 1, 1987, discloses a method and apparatus that can be used to increase the uniformity of this coating operation. This completes the printing operation, and the operator deactivates both vacuums and removes the substrate from the printing nest. Before loading the next substrate, the operator must wipe away excess conductive material that has been pulled through the through holes of the previous substrate onto the surface of the nest. Otherwise, this excess conductive material on the surface of the nest will smear onto the bottom surface of the next substrate and cause electrical shorts between conductive paths on the lower surface.
U.S. Pat. No. 5,080,921 issued to Zachman et al on Jan. 14, 1992, and assigned to the assignee of the present invention, discloses a method of avoiding the smearing problem associated the above-described manual through hole substrate printing operation. The apparatus of that invention includes a means for supporting an edge portion of the substrate and a means for interacting with the bottom surface of an interior portion of the substrate, the interacting means being located along side the support means. The interacting means, which in terms of the art could be considered to be an inner nest, includes holes of the same diameter as the holes in the substrate and positioned to directly align with the substrate holes. The apparatus further includes a means for reciprocably moving the interacting means generally perpendicular to the substrate between a position in which the interacting means is spaced from the bottom surface of the substrate and a position in which the interacting means is in a juxtaposition with the bottom surface of the substrate. The apparatus also includes a means for applying conductive material to the top surface of the substrate and a means for applying a vacuum to the holes in the interacting means to pull the conductive material through the through holes in the substrate, whereby printing occurs. The moving means junction to move the interacting means into juxtaposition with the bottom surface of the substrate when the conductive material is applied to the top surface of the substrate, and moves the interacting means to its position spaced away from the substrate after the conductive material has been applied and pulled through the substrate by the vacuum means. Thus, the substrate is in a stationary position when the printing nest is moved into juxtaposition with respect to the bottom surface of the substrate, and no sliding of the substrate with respect to the printing nest occurs. Thus, that apparatus avoids the problem of smearing of the conductive material described in the manual operation as set forth above.
The shortcomings of that apparatus include the lack of flexibility to print various conductive patterns with a single printing nest. The vacuum holes in the printing nest of that apparatus must align with the through holes in the substrate. Thus, substrates having through holes in different locations than the through holes in nest could not be printed on the same printing nest.
The present invention provides an alternative apparatus and method of through hole printing which avoids the smearing problem and avoids the need for complicated apparatus having a means for moving interacting printing nest from a position juxtaposed to the substrate into a position spaced away from the substrate.